This invention relates to a filtered cathodic arc source. More specifically, this invention relates to an improved filtered cathodic arc for generating a plasma beam containing positive ions for depositing a coating of the positive ions on a substrate. The invention relates to improved filtering of the plasma beam, to a method and apparatus for coating substrates using a filtered cathodic arc, to a method and apparatus for generating multi-layer coatings using a filtered cathodic arc, to ignition of the filtered cathodic arc, and to substrates having a coating of positive ions obtained using a filtered cathodic arc.
Various methods and apparatus are known in the art for obtaining a thin film or thin coating on a substrate. It is known to deposit films by physical vapour deposition techniques and chemical vapour deposition techniques; this invention relates to physical techniques. It is known to provide such coatings using magnetron sputtering, and one such method is described in U.S. Pat. No. 5,225,057. The quality of these films is open to considerable improvement. Sputtering is sometimes ion beam assisted. The purpose of the ion beam may be to clean substrate prior to coating or to promote reaction of subsequent deposited layers.
Another method of depositing thin films involves the use of positive ions generated from a cathodic arc source. The cathodic arc is a form of electrical discharge in vacuum which is sustained in metal plasma created by the arc alone and does not require the addition of an inert gas. Currents used in cathodic arc systems are typically of the order of 100 amps, at around 30 volts. A large percentage of the metal vapour generated by the arc is ionised by the discharge and a fraction of the arc current escapes as a beam of positive ions; this fraction is steered and optionally filtered to produce a coating on a distal substrate. The increased energy of these positive ions compared to the particles in previous deposition methods is thought to be a reason why arc evaporation techniques would deposit high density, high uniformity films. Deposition of thin films by filtered arc evaporation is described generally by P. J. Martin in Surface and Coatings Technology, Volumes 54/55 (1992) pages 136-142, and further reviewed in Surface Engineering, Volume 9 (1993), no. 1, pages 51-57.
Cathodic vacuum arc systems have thus been recognized as potentially a cost-effective method to produce coatings in a vacuum. However, the approaches taken do not address the requirements needed for the industrial applications of this technique. Such industrial system must be automatic, easily maintained and produce a large coating area, free of blemishes.
It has been observed in the deposition of films using cathodic arc technology that the plasma beam of positive ions and electrons produced by the arc is frequently contaminated by large, typically neutral, particles that are multi-atom clusters. These contaminating particles are commonly referred to as macroparticles and can be defined as particles visible under the optical microscope in a film deposited using cathodic arc methods. The presence of macroparticles in deposited films has precluded the use of cathodic arc techniques for obtaining optical and electronic coatings.
Much work in the art has been directed towards filtering macroparticles from the plasma beam, thereby eliminating the undesirable side effects of the presence of macroparticles in the deposited coating. GB-A-2117610 uses a baffle placed directly between the cathode and the substrate to prevent macroparticles reaching the substrate. The positive ions in the plasma beam are focused around the baffle. This has the disadvantage that there is a very low transmission of the plasma beam to the substrate. Further, some macroparticles reach the substrate by bouncing off the sides of the apparatus.
U.S. Pat. No. 5,279,723, in the name of Falabella et al, describes a cathodic ion source in which the plasma beam is filtered in an attempt to eliminate macroparticles by providing a bent magnetic field to guide ions in the plasma beam around a 45.degree. bend, there being no line-of-sight from the arc spot to the substrate. U.S. Pat. No. 5,279,723 also describes using permanently fixed baffles in the plasma duct to trap macroparticles. These confer the disadvantage that they quickly become covered with a dense coating of macroparticles which can fall back into the plasma beam and therefore lead to contamination of the substrate coating. Cleaning of these baffles is awkward as it must be carried out on a partially disassembled cathodic arc source. A similar shaped duct is described in U.S. Pat. No. 5,433,836.
Neither apparatus entirely prevents macroparticles from the plasma beam arriving at the substrate; it is indicated that tests of U.S. Pat. No. 5,279,723 show a figure of less than 0.2 macroparticles/cm.sup.2 /minute of coating time for this apparatus. This latter figure, however, is not accompanied by any precise deposition conditions thus preventing any confident assessment of the filtering efficiency of the apparatus described.
Other known apparatus use a 90.degree. bend duct with an axial magnetic field to filter out the macroparticles. Although these may achieve acceptable results, they do not provide a long term solution for industrial equipment designed for near-continuous working. The generous amount of materials coated onto the duct wall build up over time and may lead to particles later being resputtered back into the plasma flux. Thus, only short periods of use of the equipment is possible. Also, deposited films still contain significant macroparticle contamination.
A third U.S. Pat. No. 5,401,543, describes using a particular cathode target material, but still suffers from high levels of macroparticles in deposited films.
The number of macroparticles in the plasma beam can be reduced by increasing the pressure in the vacuum chamber. However, an increase in pressure in the vacuum chamber is likely to lead to a deterioration in the properties of the layer deposited on the substrate. When depositing tetrahedral amorphous carbon, a trace amount of H.sub.2 can reduce macroparticles and increase transparency in the visible light range but will compromise the density and hardness of the deposited layer, by gas inclusion in the deposited film.
It is therefore a problem in the art to generate a filtered plasma beam from a cathodic arc source, which filtered beam is substantially free of macroparticles.
Another related problem concerns a tool that is needed to ignite the arc. Once ignited, the tool is no longer required until arc re-ignition. Initiation of the arc has been typically determined visually, whereby a graphite rod fixed to a grounded stainless steel plunger is manually manoeuvred against the cathode and in which the movement is observed via an observation window in the wall of the vacuum chamber. This arrangement is particularly awkward, especially as the window is rapidly coated and becomes opaque. Alternatively, a fixed tungsten or graphite trigger mounted close to the cathode surface is used to ignite the arc by the application of a high voltage. Due to this proximity to the cathode, the elements of the igniter tend to contaminate the plasma beam.
Another problem in the art is that no satisfactory method of obtaining multi-layer coatings is described. At present, to obtain a multi-layer coating on a substrate requires either two separate coating machines, one for producing each individual layer of the multi-layer coating, or the depositing of a first layer using one coating machine and then the dismantling of this machine so as to replace the cathode and then reassembly of the machine to deposit the second layer of the multi-layer coating.
Where the coating is a composite of metal and gas atoms, such as aluminium oxide or silicon dioxide coatings, this is achieved in the art by creating a plasma beam of metal ions, depositing these on the substrate and reacting the deposited ions with the gaseous component, such as oxygen, of the coating. These prior art techniques are rather inefficient in that they use large volumes of gas. An improved method of producing coatings of this nature is required.